Necrotizing arteritis mimicking polyarteritis nodosa occurred in transgenic rats carrying the env-pX gene of human T-cell leukemia virus type I. To investigate the pathogenesis of necrotizing arteritis in these rats (envpX rats), adoptive transfers of spleen cells and bone marrow cells were done from env-pX rats before they developed arteritis to nontransgenic rats. Necrotizing arteritis occurred in lethally irradiated nontransgenic rats reconstituted by env-pX spleen cells, thus indicating that the env-pX transgene in affected vessels may not be essential for the development of arteritis. In contrast, arteritis was not induced in nontransgenic recipients by adoptive transfers of env-pX bone marrow cells, which suggested that T cells derived from the env-pX thymus may play a role in the development of arteritis. To clarify if the process of differentiation of T cells in the env-pX thymus is crucial to develop necrotizing arteritis, reciprocal exchange of thymus frameworks was done between env-pX and nontransgenic rats. Necrotizing arteritis occurred in nontransgenic rats with an env-pX thymus framework, whereas development of arteritis was suppressed in env-pX rats in which the thymus framework was replaced with a nontransgenic one. This collective evidence shows that the thymus is directly associated with the development of necrotizing arteritis in env-pX rats. Human T-cell leukemia virus type-I (HTLV-I) is associated with a number of diseases, including adult T cell leukemia, 1,2 myelopathy, 3,4 uveitis, 5 and probably arthropathy, 6 Sjö gren's syndrome, 7 T cell alveolitis, 7,8 and infective dermatitis. 9 To investigate the pathogenetic role of HTLV-I in these diseases, we established and analyzed rat models for HTLV-I infection 10 -16 and transgenic rat lines carrying the env and/or pX genes of HTLV-I. [17][18][19][20][21] In our previous work, the env-pX gene was introduced into the germline of Wistar-King-Aptekman-Hokudai (WKAH) rats under the control of the HTLV-I LTR promoter. 18 The env-pX transgene was expressed constitutively in systemic organs of these rats (env-pX rats). Vasculitis and other collagen vascular diseases occurred in env-pX rats. Small-to medium-sized arteries in connective tissues around the thymus, salivary glands, testis, pancreas, and in the dermis, but never glomeruli or large vessels such as the aorta, were mainly affected. A cellular infiltrate containing lymphocytes, neutrophils, plasma cells, and histiocytes with proliferation of fibroblastic cells surrounding affected arteries was present, and fibrinoid necrosis was evident in the intima and/or media. These histological characteristics corresponding to necrotizing arteritis and the tissue distribution resemble findings in polyarteritis nodosa in humans. Because autoantibodies, including anti-nuclear and anti-DNA antibodies, were present in the sera, autoimmune phenomena may play a role in the pathogenesis.To determine whether cellular autoimmune responses are involved in the pathogenesis of necrotizing arteritis in env-pX rats...